fix(es/typescript): Handle multiline type parameters in async arrow f…

…unctions (#9704)

**Related issue:**

- Closes #9698

.changeset/swift-hairs-bow.md

@@ -0,0 +1,6 @@
+---
+swc_fast_ts_strip: patch
+swc_core: patch
+---
+
+fix(es/typescript): Handle multiline type parameters in async arrow functions

crates/swc_fast_ts_strip/src/lib.rs

@@ -555,6 +555,34 @@ impl Visit for TsStrip {
     }
 
     fn visit_arrow_expr(&mut self, n: &ArrowExpr) {
+        #[inline(always)]
+        fn is_new_line(c: char) -> bool {
+            matches!(c, '\u{000A}' | '\u{000D}' | '\u{2028}' | '\u{2029}')
+        }
+
+        'type_params: {
+            if let Some(tp) = &n.type_params {
+                self.add_replacement(tp.span);
+
+                if !n.is_async {
+                    break 'type_params;
+                }
+
+                let slice = self.get_src_slice(tp.span);
+                if !slice.chars().any(is_new_line) {
+                    break 'type_params;
+                }
+
+                let l_paren = self.get_next_token(tp.span.hi);
+                debug_assert_eq!(l_paren.token, Token::LParen);
+                let l_paren_pos = l_paren.span.lo;
+                let l_lt_pos = tp.span.lo;
+
+                self.add_overwrite(l_paren_pos, b' ');
+                self.add_overwrite(l_lt_pos, b'(');
+            }
+        }
+
         if let Some(ret) = &n.return_type {
             self.add_replacement(ret.span);
 
@@ -565,10 +593,7 @@ impl Visit for TsStrip {
             let span = span(r_paren.span.lo, arrow.span.lo);
 
             let slice = self.get_src_slice(span);
-            if slice
-                .chars()
-                .any(|c| matches!(c, '\u{000A}' | '\u{000D}' | '\u{2028}' | '\u{2029}'))
-            {
+            if slice.chars().any(is_new_line) {
                 self.add_replacement(r_paren.span);
 
                 // Instead of moving the arrow mark, we shift the right parenthesis to the next
@@ -597,7 +622,6 @@ impl Visit for TsStrip {
             }
         }
 
-        n.type_params.visit_with(self);
         n.params.visit_with(self);
         n.body.visit_with(self);
     }

crates/swc_fast_ts_strip/tests/fixture/issue-9698.js

@@ -0,0 +1,9 @@
+let f = async (
+
+  v   ) => v;
+
+let g = async (
+
+  v    
+                 ) => 
+        v;

crates/swc_fast_ts_strip/tests/fixture/issue-9698.transform.js

@@ -0,0 +1,2 @@
+let f = async (v)=>v;
+let g = async (v)=>v;

crates/swc_fast_ts_strip/tests/fixture/issue-9698.ts

@@ -0,0 +1,9 @@
+let f = async <
+    T
+>(v: T) => v;
+
+let g = async <
+    T
+>(v: T)
+    : Promise<any> => 
+        v;